1. Field of the Invention
The present invention relates to a steel cord for rubber reinforcement, which is buried in a rubber product, such as a tire, and more particularly, to a steel cord for rubber reinforcement and a method for manufacturing the same, in which an infinitesimal amount of a cobalt compound is added to a wet lubricant at the time of drawing a brass-plated steel wire that forms the steel cord so that brass and cobalt are alloyed with each other in the top-surface of the brass plating layer, thereby achieving an aging adhesion improvement of the steel cord.
2. Description of Prior Art
A steel cord, buried in a vehicle tire to be used for rubber reinforcement, includes a brass plating layer fainted on the surface thereof so as to improve adhesion with the tire rubber and drawing machinability in a steel wire manufacturing process. The steel wire including the brass plating layer formed in this way is buried in a tire as a single body or in a stranded state where multiple steel wires are stranded, so as to reinforce the tire.
Meanwhile, the adhesive strength between the brass-plated steel wire and the tire rubber is gradually reduced as time passes as compared to the adhesive strength at the early stage of vulcanization due to various factors. The representative adhesive strength reducing factors may include intensive heat and moisture situations suffered by the tire while a vehicle is traveling.
First, descriptions will be made on the heat according to the traveling of the vehicle. As the temperature of tires rises while the vehicle travels at a high speed, sulfur, which did not fully undergo vulcanization at the early stage of vulcanization, continuously causes vulcanization and increases hardness of the rubber so that the rubber itself losses elasticity. Then, fatigue degradation is caused by the impact continuously applied from the road and the weight of the vehicle. In addition, the heat generated during the traveling causes an adhesive reaction between the brass and the rubber so that a copper sulfide layer produced at the early state of vulcanization is continuously produced. The copper sulfide layer grown over a normal thickness is easily fractured from the brass layer by the impact applied to the tire, which causes the reduction of adhesive strength.
Next, descriptions will be made on the moisture. When the tire rubber is damaged, water permeates into the tire rubber through the damaged portion so that chemical breakdown and corrosion occur around the steel cord, which causes a sharp reduction of the initial adhesive strength produced at the time of vulcanization.
Accordingly, in order to extend the life span of the tire, providing high heat-resistant adhesion and water-resistant adhesion is considered as important as maintaining the high initial adhesive strength between the plated steel wires and the tire rubber.
As a measure for improving the heat-resistant (corrosion-resistant) and water-resistant adhesion as important quality characteristics requested for the steel cord, there is known a method of forming a plating layer of a ternary alloy or a quaternary alloy by adding other elements to the brass that forms the surface of the steel wire.
For example, Korean Patent Laid-Open Publication Nos. 2000-0074219 and 1995-0000929 disclose a method of obtaining a plating layer formed of a ternary alloy of Cu—Zn—Co by sequentially plating copper, zinc, and cobalt on the surface of a steel wire, and performing a diffusion step. However, such a method requires separate additional steps to plate and diffuse a third element, cobalt, in addition to the brass plating layer forming step, which unavoidably causes the complication of the manufacturing process and the increase of manufacturing costs.
Meanwhile, for example, Korean Patent Laid-Open Publication No. 1993-0013214 and Japanese Patent Laid-Open Publication No. 2003-171887 disclose a technique for improving the corrosion-resistant and water-resistant adhesive strength of a steel cord merely by coating a cobalt compound on a steel wire surface during a drawing or elongation step of a brass-plated steel wire. However, the cobalt compound merely coated on the surface of the steel cord (plated steel wire) is not strongly bonded with the brass layer, thereby merely suppressing the reduction of the water-resistant adhesion of the brass in relation to only the rubber, rather than having an effect on the adhesion interface layer of the brass and the rubber so that the practical effect is not so high.
For example, Korean Patent Laid-Open Publication Nos. 2001-0003864 and 2008-0072700 disclose a method for improving corrosion-resistant and water-resistant adhesion by providing a separate lubricating bath, in which a cobalt compound is dissolved, outside the outlet of a drawing bath at the time of drawing so that the cobalt compound, coated on the surface of the brass-plated steel wire passing through the lubricating path, forms a ternary alloy of brass-cobalt on the surface while passing through the final die. With the above-mentioned method, it is expected that the above-mentioned effect may be achieved at the time of small-scale production. However, as the production increases, that is, as the operating time of the lubricating bath increases, a frictional force between the dies in the lubricating bath and the steel wire increases, the temperature of the lubricating liquid in the lubricant rises due to the high-temperature heat generated from the dies, and a sharp deterioration of wire drawability is caused so that, for example, serious surface cutting or snapping may occur in the steel wire after the steel wire is drawn.
In addition, the content of the cobalt component bonded to the surface of the steel cord according to the above-mentioned method has a high concentration of several ppm or more. The high concentration of cobalt does not cause a practical problem in small-scale production. In mass production, however, due to the friction between the cobalt component used as an adhesive material at the time of sintering nibs within the dies used for drawing and the cobalt contained in the lubricant, the fracturing of dies increases and fragments chipped off from the nips of the dies may be caught in the inlet of the subsequent dies and scratch the surfaces of the drawn wires. Then, in the stranding step performed thereafter to strand the steel wires, when torsional stresses are applied to the steel wires by the stranding of the steel wires, snapping frequently occurs at the surface-scratched portions, which causes a reduction in productivity.
As no clear solution for the problems described above has been proposed to date in the related art, cobalt is included in the rubber rather than being bonded to the surface of the steel cord. That is, adhesive rubber in all the tires includes a cobalt component additive. Since a sufficient amount of cobalt is included in the adhesive rubber, the aging adhesive strength improvement effect by the cobalt component existing on the surface of the steel cord at a high concentration of a predetermined level or more is lower than might be expected.
In addition, when the cobalt component exists on the surface of the steel cord at a high concentration, the cobalt is eluted as a heavy metal when the tires, of which the lives have been ended, are disposed of, and thus, environmental pollution is caused. Thus, when the cobalt component is used for improving adhesion between the steel cord and rubber, it is necessary to apply the cobalt component within a minimal and optimal content range that is capable of removing the above-mentioned problems while answering the purpose of using the cobalt component.